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<b>BridgeDb</b> is a java library intended for dealing with identifiers for
biological entities such as genes, proteins, metabolites.
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In BridgeDb, a {@link org.bridgedb.DataSource} defines an online resource such as Ensembl or Entrez.
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An {@link org.bridgedb.Xref} is the combination of an identifier and a particular resource, 
and thus uniquely defines a single biological entity. It is often possible
to generate a URL to an online description of this biological entity.
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Identifiers are preferably not descriptive. A descriptive name such as INSR (Insulin Receptor) 
instead of Entrez Gene 3643 can be useful. In BridgeDb we refer to this as a Symbol. Because
of the ambiguous nature, Symbols are not the primary means for identifying 
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Usually, different DataSources describe the same biological entities. Cross references are
stored in so called Synonym databases. {@link org.bridgedb.IDMapper} has facilities for mapping cross-references
and mapping between different DataSources.
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